1. Field of the Invention
The present invention relates to a method and an apparatus for superimposing an additional information to a video signal in such a manner as giving influence on the reproduced video signal as least as possible.
2. Description of the Related Art
A digital information recording apparatus such as a digital VTR and a MD (Mini-disc) recording/reproducing apparatus is widely spreading and moreover a DVD (digital video disc or digital versatile disc) apparatus providing a recording function is developed at present. In such a digital information recording apparatus, a digital video signal act and digital audio signal as the main information signal and moreover various kinds of additional information signals incorporated to data for computer can be recorded.
In this case, this additional information signal is a digital signal and is added to a digital information signal as the signal to be recorded to the area which is discriminated regionally, from the digital information signal such as a header area which is added, for example, to the data in a unit of a block of the digital information signal and the other TOC (Table of Contents) area.
In the case of the existing system for recording and transmitting the main information signal by superimposing thereto an additional information, the additional information signal is not superimposed in direct to a digital information signal in the header area but is added to the indirect area. Therefore, it is likely that the additional information signal is removed rather easily because of filtering and alteration and thereby a recording apparatus and a reproducing apparatus can no longer detect the necessary additional information signal. Particularly when a control information or a copy right information to prevent unfair duplication is added as the additional information signals, the first object cannot be attained because such additional information signal is removed.
In addition, when the additional information signal is added to the indirect area as explained above, if a digital signal is converted to an analog signal, only the main information signal can be obtained, losing the additional information signal. It means that even when a measure for preventing unfair duplication of digital information signal is attempted by adding a duplication preventing control signal as the additional information signal, if the digital signal is converted to the analog signal, the measure for preventing duplication is meaningless.
As an additional information signal adding system to solve the problems as explained above that the additional information signal is removed and that a measure is no longer effective when the digital signal is converted to analog signal, the applicant of the present invention has proposed a system where the additional information signal such as duplication preventing control signal is spectrum spread, this spectrum spread additional information signal is superimposed on a video signal and this video signal is then recorded in digital or analog mode (U.S. patent application Ser. No. 08/755101).
In this system, the code of PN (Pseudorandom Noise) series (hereinafter, referred to as PN code) to be used as the spread signal is generated in sufficiently quick period, it is then multiplied to the additional information signal for the purpose of spectrum spread in view of converting the additional information signal such as narrow band and high level duplication preventing control signal into a broad band and low level signal which does not give any influence on the video signal. This spectrum spread additional information signal, namely, the spectrum spread signal is superimposed on the analog video signal for the recording on a recording medium. In this case, the video signal to be recorded on the recording medium may be analog signal or digital signal.
In this system, since the additional information signal such as the duplication preventing control signal is superimposed in the same period and same frequency as the video signal, it is difficult for a person who is trying to conduct illegal duplication, for example, to remove the superimposed duplication preventing control signal from the video signal. On the other hand, it is possible to detect and use the superimposed additional information signal such as duplication preventing control signal by the inverse spectrum spread.
As explained above, the additional information to be superimposed on the video signal must be superimposed to the video signal in a very low level not to give any influence on the reproduced image of the video signal. However, the additional information superimposed in a very low level is also required to have the level which can be detected and such level must be superimposed in the level as high as possible for the purpose of detection.
It is therefore an object of the present invention to provide a method and an apparatus for superimposing the additional signal to the video signal in the level as high as possible for easy detection without giving influence on the reproduced image of the video signal.
In the method and apparatus of the present invention for superimposing an additional information to a video signal, a spectrum spread code having the chip section corresponding to the period of one image or a plurality of pixels of the video signal is generated in synchronization with the video signal, the additional information signal is spectrum spread by the spectrum spread code to generate the spectrum spread additional information signal, thereby the edge portion of the image of the video signal is detected, when the edge portion is detected, a level changed spectrum spread additional information signal in which the level of the spectrum spread additional information signal part corresponding to the edge portion is set larger than the other portion is generated and the level changed spectrum spread additional information signal is superimposed to the video signal to generate an output video signal.
The edge portion of the video signal changes its pixel level to a large extent and when the spectrum spread additional, information signal is superimposed to this edge portion in the higher level than the other portion, it is not distinctive on the reproduced image. As explained above, since the superimposing level of the additional information signal can be raised, the additional information signal can be detected easily.
The superimposing level is determined depending on the level difference of the edge portion and therefore deterioration of the reproduced image due to the video signal can be prevented as much as possible and moreover the superimposing level as high as possible can also be set.